Buying the miles is only ~$200; given that you want to redeem for a ~$10k ticket, is it worth taking the risk that there won't be any availability when you have finally earned the 8k ? If there is the BA award availability you want available now, I'd bite the bullet and buy the miles. After all, that's still less than the taxes and Fees you pay flying to/from LHR.

OTOH, you could always buy some fully refundable tix for early next year (buy online with the CC, and don't forget to book them as 1-ways for the extra bonus), book the award now, and then cancel after you have earned the 8k miles that they'll take back from you when you do. 3 R/Ts at ~$650 each should do it. Of course you'll be lending AS ~$2k interest free....

Some might regard this as unethical, and I wouldn't do it, but it is an option.